使用多处理时，全局变量未在python中更新

Question

I have a following simple code

from multiprocessing import Pool
x = []


def func(a):
    print(x,a)


def main():
    a = [1,2,3,4,5]
    pool = Pool(1)
    global x
    x = [1,2,3,4]
    ans = pool.map(func,a)
    print(x)

It gives me the result

[] 1
[] 2
[] 3
[] 4
[] 5
[1, 2, 3, 4]

I expected the result to reflects the change in global variable x. Which seems that the changed in global variable x is not updated before the pool call. I would like to ask what is the cause of this?

Answer 1

So I have done what GuangshengZuo suggested, and sadly the result was not desirable. After looking deeper into it, I realized the problem was not because of script, but rather the OS.

In windows, there is no os.fork() , hence the change in global variable is not copied. But, on Unix machine, the script works fine.

Answer 2

Two seperate processes will not share the same global variables. A multiprocessing pool abstracts away the fact that you are using two seperate processes which makes this tough to recognise.

Answer 3

I think it is because this is multiprocess, not multithread. the main process and the new process does not share a same global variable. So the new process has the copy of the main process when x is [], and after created, main process change x's value, but it does not change to new process's x.

if change the code to this :

from multiprocessing import Pool
x = []


def func(a):
    print(x,a)


def main():
    a = [1,2,3,4,5]
    global x
    x = [1,2,3,4]
    pool = Pool(1)
    ans = pool.map(func,a)
    print(x)

and the ouput will be what you want. Notice the pool = Pool(1) 's position